Juvenile hemochromatosis is a severe variant of hemochromatosis that is caused by mutations in two genes which give indistinguishable phenotypes. One gene encodes hepcidin (HAMP, 19q13.1), a peptide synthesized by the liver in response to inflammation and iron overload. The second gene has recently been identified as hemojuvelin (HJV, 1q21). Although the function of HJV is unknown, hepcidin levels are depressed in persons with HJV mutations, indicating that HJV may be a hepcidin modulator. HJV is also a member of the RGM family of proteins originally described by our group and others as neural adhesion molecules. We present preliminary studies in vitro in liver cells that 1) HJV can induce BMP but not TGF-beta signals, 2) HJV signaling can be blocked by Noggin, a well-known BMP inhibitor, 3) HJV can bind directly to radiolabeled BMP-2 ligand, 4) HJV signals via the BMP type I receptors, ALK3 and ALK6, 5) HJV signals via the R-Smad, Smad1, 6) an HJV mutant known to cause juvenile hemochromatosis has decreased BMP signaling ability, and 7) BMP can increase, while Noggin can decrease, hepcidin expression in liver cells. We believe that HJV is a novel BMP co-receptor whose BMP signaling ability is important in regulating iron metabolism. Loss of BMP signaling by mutations in HJV could lead to decreased BMP signaling in liver cells, which would cause decreased hepcidin expression, and would thus explain why persons with HJV mutations also have depressed hepcidin levels. We propose to: 1) Determine the molecular mechanism of HJV action in the BMP signaling pathway in liver cells in vitro, including characterizing the physical interaction of HJV with the known BMP receptors and determining the binding affinities of HJV for different BMP ligands; 2) Study the role of key structural features of HJV on its function, including HJV's RGD motif, GPI anchor and its proteolytic cleavage site; 3) Determine the role of HJV and BMP signaling in iron metabolism in vitro and in vivo in HJV knock-out mice. Knowledge gained regarding the mechanism of action of HJV would be important in furthering our understanding of BMP signaling and of iron metabolism, and could lead to novel treatments of disorders of iron metabolism such as hemochromatosis and anemias of chronic disease.